FWIW I can remember only two places where that might be useful - undo/redo mechanism in re-frame and a similar thing but in re-frame-10x. But implementing something like that using your mechanism would also require an ability to circumvent the mechanism - i.e. to rollback from 3 to 1 above, you want to both reset counter to 1 and to reset counts to [0 1].

yeah it's not really useful for undo/redo

i think it could be useful for relatively static higher order operations though, like filtering values

(def evens
  (r/reduce
    (fn [prev v]
      (if (even? v) v prev))
    0 counter))

normally you would have to maintain a list of all the counts in the source, filter the list then take the first. in this case the source can be a single value and we filter each individual value as it's propagated

How is what you propose different from just using add-watch and swapping an extra ratom in there?

you can add dependent reactions to it

(re-frame/reg-sub ::counter #(:counter db))

(defn only-evens
  [prev v]
  (if (even? v) v prev))

(def evens (r/reduce only-evens 0 (re-frame/subscribe ::counter))

(defn counter-view
  []
  [:div
   [:div "Count: " @(re-frame/subscribe ::counter)]
   [:div "Even count: " @evens]])

you could also cut off calculations using reduced for performance

Funny you should mention re-frame - in that setting, I would definitely try to avoid r/reduce. :) And if I think about it outside of those preferences, I'm still not entirely convinced such a [relatively] complicated thing justifies the value it brings. But maybe someone else might chime in and provide more useful commentary.

why would you avoid r/reduce in a re-frame context?

Because it's an implicitly derived state without any options on undo/redo.

If I needed some sort of a reduced accumulator, I'd definitely store both the accumulator and the last value fed into the reducing function in app-db itself.

the reason is because you want to have the ability to undo/redo?

Undo/redo, stable code reloading, saving and restoring state, central storage for monitoring - all the nice things that follow from the re-frame model.

good points